1. Field of the Invention
The present invention relates to an optical glass suitable for precision press-molding and a process for producing an optical element (such as a lens) made of the optical glass.
2. Discussion of Background
Recording on an optical recording medium such as CD is carried out by making a laser beam a parallel ray by a collimating lens and condensing the ray by an object lens.
The lens to be used for such an application is usually spherical or aspherical, and is prepared by heating a glass or resin preform to a softening temperature, followed by precision press-molding.
In a case where a lens is prepared by subjecting glass to precision press-molding, a glass preform is prepared usually by dropping a molten glass from a flow outlet of a flow out pipe made of platinum or a platinum alloy (hereinafter they will generically be referred to as platinum or the like). In such a case, a phenomenon such that the molten glass is wetly attached to the outside of the end of the flow outlet of the flow out pipe, and intensified striae of the preform and an increase in the fluctuation of mass of the preform by the phenomenon, are known.
Paying attention to that the above phenomenon is attributable to a small contact angle between the molten glass and platinum or the like, a glass with which the contact angle can be increased has been proposed (Japanese Patent No. 3270022).
In recent years, reduction in size of glass lenses produced by precision press-molding is increasingly in progress. As a result, a preform to be used for production of an object lens for pickup for DVD has an extremely light weight at a level of several dozen mg to several hundred mg. Accordingly, the diameter at the end of the flow outlet becomes smaller, and the above phenomenon that the molten glass is wetly attached to the end becomes remarkable, and the fluctuation of mass of the preform further increase, such being problematic.
However, the above mentioned glass with which the contact angle can be increased (hereinafter referred to as conventional glass) is not sufficient to dissolve the above problems, and a glass with which the contact angle can be more increased has been desired.
Further, as a lens to be used for e.g. DVD employing a blue laser (hereinafter referred to as blue DVD), a lens having a high refractive index, a large Abbe number and a high transmittance to a light having a wavelength of 400 nm, has been desired.
If a lens has a low refractive index, the numerical aperture tends to be small, and when the lens is used as an object lens for example, the distance between the lens and the surface of a recording medium tends to be very short, whereby it is very possible that the lens and the recording medium are in contact.
Further, if a lens has a small Abbe number, the wavelength dispersion tends to be remarkable, and the light may not be focused at the time of the laser wavelength drift.
Further, if the above transmittance of a lens is low, the laser beam intensity to be applied to a recording portion of a recording medium tends to be insufficient and the irradiation time has to be long, and the reading and writing rates may be thereby low.
However, the conventional glass contains SiO2 and B2O3 and is characterized in that the content ratio SiO2/B2O3 as represented by mass percentage is higher than 0.78, and is unsatisfactory for meeting the above requirements. That is, since it contains SiO2 and thereby has a high glass transition point (Tg), the Li2O content has to be increased so that Tg will not be high, and as a result, problems may arise such that its refractive index will decrease or it is likely to be devitrified. Further, if TiO2 or Nb2O5 is added so as to increase the refractive index without increasing Tg, a problem may arise such that the transmittance at a wavelength in the vicinity of 400 nm will decrease.